1. Field of the Invention
This invention relates generally to active and passive opto-electronic and photonic devices that include an optical waveguide device. This invention particularly relates to an optical waveguide device which has an optical waveguide for guiding a wave therethrough and a grating coupler disposed on the surface of or into the optical waveguide such that the guided optical wave may be radiated by the grating coupler out of the optical waveguide or an external optical wave may be introduced by the grating coupler into the optical waveguide and which may include a semiconductor diode laser device and/or a semiconductor detector.
2. Description of the Prior Art
In many applications it is required that the grating couplers efficiently radiate most of the guided electromagnetic power into a desired radiation beam within a short coupling length. For example, in a focusing grating coupler used for CD pick-up head, it is not only essential to focus most of the radiated photonic: power into a spot, but also necessary to suppress the transmitted guided photonic power remaining in the optical waveguide and spurious radiation beam(s) which will otherwise contribute to the loss of power.
It is well-known that within a waveguide grating region the guided wave, P.sub.g (z), is an exponentially decaying electromagnetic wave along the propagation direction EQU P.sub.g (z)=P.sub.o exp(-az), (0&lt;z&lt;L) (1)
where z is the propagation direction, P.sub.o is the incident guided power at z=0, a is the power radiation factor, and L is the total length of the grating. The grating radiation directionality (`Branching Ratio`), R, is defined as the percentage of the total radiation power directed into a desired radiation beam and the output coupling efficiency of the grating couplers is: EQU .eta.=R [1-exp(-aL)].times.100% (2)
From Eq.(2) it occurred to us that the second term denotes the total amount of the incident guided power being radiated by the grating and that, for a given grating length L, both a large radiation directionality R and a large radiation factor a are required for high-efficiency output coupling.
Originally, grating couplers used symmetrical tooth profiles (e.g. rectangular, and sinusoidal etc.), such profiles did not present good radiation directionality. Subsequently, as proposed by S. T. Peng and T. Tarnir in "Directional blazing of waves guided by asymmetrical dielectric gratings", Optics Communications, 11, 405-409 (1974), asymmetrical (or blazed) gratings were used for optical waveguide structures in an attempt to scatter the modal energy preferentially. However, all such previous asymmetrical designs concentrated on the sawtooth (trapezoidal or triangular) grating tooth profiles. Unfortunately this type of grating had the following problems: (1) although a large radiation directionality is available, the radiation factor is reduced; (2) due to the geometric constraint as discussed below in connection with FIG. 3, the blaze angle (or radiational directionality R) and the grating depth (or radiation factor a) can not be independently controlled. These problems restrict the application of the sawtooth gratings.
For input coupling, however, we found that a third term called the beam overlap coefficient, M(g,h) must be included in Eq.(2) in order to determine the input coupling efficiency. Referring to Eq.(3), g(z) is the complex amplitude distribution of the radiation beam arising from the output coupling, h(z) is the complex amplitude distribution along the grating of the input beam, and M(g,h) is defined as ##EQU1## where M(g,h) is less than unity. Generally, the input coupling efficiency is lower than the corresponding output coupling efficiency; attributed to the dissimilarity between the radiation and the input beam.
To increase the input coupling efficiency, we found that it is therefore necessary to shape the radiation beam profile so as to increase the similarity between the radiation and the input beam, i.e. M(g,h). Although other investigators attempted to modify the beam profile, for example, by varying the grating tooth depth along the guided wave propagation direction as described in U.S. Pat. No. 5,101,459, issued Mar. 31, 1992, to H. Sunagawa, such techniques were not fully compatible with the standard VLSI dry etching technology, since a uniform etching depth is generally obtained within a large area. In addition, Sunagawa's device could not simultaneously control R and a.
On the other hand, we discovered that both R and a can be simultaneously optimized using our invention. Further background is provided in our article in Optics Communications 109 (1994) pp239-245.